(Not applicable)
1. Field of the Invention
The invention relates to electronic devices which use microphones to convert sound signals to electrical signals.
2. Description of Related Art
Microphone units typically operate in environments where unwanted noise is present. For example, a person listening to someone talking on the telephone may be distracted from the speaker""s voice by sounds emanating from machinery, traffic, appliances, or other ambient sounds, if the person is talking into a phone without a noise-cancelling microphone.
Many noise-cancelling microphone element designs employ front and rear sound ports which allow sound to enter both and impinge upon the diaphragm simultaneously in opposite directions resulting in little r no signal being generated by the microphone. This technique is applied in a wide variety of cardioid microphones as well as telephone handset transmitters and headsets. Some employ acoustic tuning to the rear port to make it more frequency responsive.
Noise-cancelling microphones depend upon two factors for their operation. The first factor is the polar pattern of the microphone (usually bi-directional) and the assumption that the noise to be reduced is not on the maximum sensitivity axis of the microphone. The second factor is the different responses of the bi-directional microphone for a sound source close to the microphone (i.e., entering the front sound port) and a sound source at a distance to the microphone (i.e., entering the front and rear sound port).
When the sound source is close o the front sound port of the microphone, the sound pressure will be several times greater at the front than at the rear. Since the microphone responds to the difference of sound pressure at the two entries, close talking will provide a substantially higher sensitivity than a remote sound, where the sound pressure is equal in magnitude at the two entries.
Because of construction restraints inherent in front and rear sound port microphone design, one port of the microphone is always more sensitive. This results from the need to provide a supporting structure for the diaphragm and the resulting impedance that structure presents to sound entering the rear sound port microphone element. In common practice, the more sensitive port is faced forward to capture the desired sound while the less sensitive port is utilized for capturing and nulling the undesired background noises.
If the front and back sensitivities of the element were equal, then theoretically 100% noise rejection would be possible whenever noise of equal pressure is subjected to both entrances to the microphone. In practice however, only 10-20 dB noise reduction is possible using the currently available microphone elements and this is only for frequencies below about 3 KHz.
Frequency response is another factor that differentiates noise-cancelling microphones. Frequency response is essentially flat in the near field (i.e., a sound source close to the front sound port) over the audio band. In the far field (i.e., a remote sound source), the frequency response increases with frequency until the pressures at the front and rear ports of the unit are 180 degrees out of phase at which point resonance occurs. At some frequency, the microphone becomes more sensitive to axial far field sounds than axial near field sounds. This crossover frequency will occur at a higher frequency for a microphone with a shorter port separation than a microphone with a longer port separation.
Several devices, both electrical and mechanical, used for noise-cancellation exist but have potential drawbacks such as the need for preprocessing, effects of reflections, calibration difficulties, cost, and operating environment. For example, in environments in which human speech is the ambient noise, signal processing techniques such as filtering can not effectively be used because the ambient human speech is at the same frequency as the desired speaker""s voice and because the ambient noise is non-constant or non-periodic.
U.S. Pat. No. 5,854,848 to Tate, et al., assigned to the same assignee of record and incorporated by reference herein in its entirety teaches the use of a specific deflector configuration to mechanically direct sound to a differential microphone (that is, a bi-directional microphone). The system of the ""848 patent enhances the performance of pressure differential microphones used to cancel or reject background noise. When a pressure differential microphone and deflectors of the ""848 patent are used together they form an electroacoustic noise rejection system exceeding the performance of other commercially available technologies.
The system of the ""848 patent effects a high degree of cancellation of the impingement of ambient noise upon the front surface of a pressure differential microphone by directing the same ambient noise upon the back side of the microphone. The system of ""848 patent causes ambient noise (including voice, non-constant noise, non-periodic noise, and random noise) to enter the microphone on both sides simultaneously and with the strength of the sound on the back side relatively higher slightly to overcome the relatively higher impedance of the back side of the microphone, thus nullifying the effect of the noise sound waves. Furthermore, the system of the ""848 patent deflects the talker""s voice (i.e., tie desired sound to be transmitted) away from the back side of the microphone.
The system of the ""848 patent utilizes curved reflectors to direct ambient noise into the back side of the microphone even when the rear port of the microphone is not aligned with the source of greatest ambient noise. In addition, the sound pressure of the ambient noise entering the back side of the microphone is increased by the curved reflectors being larger than the opening leading to the back side of the microphone. By such a configuration, ambient noise sound waves entering the front of the microphone are cancelled at the microphone by the same ambient noise converging upon the back surface of the microphone. The curved reflectors also act to deflect the speaking voice away from the back side of the microphone so that the speaker""s voice enters the front side of the microphone only. This is essentially to prevent self-cancellation.
In one aspect, the system of the ""848 patent provides a noise-controlling apparatus for use with a directional microphone having a housing having a first sound opening located in a front side of a barrier element and a second sound opening located in a back side of the barrier element. The housing having a curved reflector extending from the back side of the barrier element which deflects a user""s voice away from the second sound opening and deflects ambient noise toward the second sound opening.
In another aspect, the system of the ""848 patent provides a noise-controlling apparatus having a microphone having both a sound-receiving front side and a sound-receiving back side. The housing having a centrally located barrier element with a first sound opening in a front side of the barrier element and a second sound opening in a back side of the barrier element communicating with the sound-receiving front and back side, respectively, of the microphone. The housing having a first curved reflector and a second curved reflector each extending from the back side of the barrier element and which deflect a user""s voice away front the second sound opening and ambient noise toward the second sound opening.
In yet another aspect, the system of the ""848 patent provides a noise-controlling apparatus having a microphone having a sound-receiving front side and a sound-receiving back side. The housing having a centrally located barrier element with a first sound opening in a front side of the barrier element and a second sound opening in a back side of the barrier element communicating with the sound-receiving front and back side, respectively, of the microphone and portions for deflecting a user""s voice away from the second sound opening and deflecting ambient noise toward the second sound opening.
The system of the ""848 patent efficiently controls ambient noise and improves microphone performance. When built into conventional electronic sound receiving devices and coupled with the electronic circuitry thereof, device performance is greatly enhanced in a relatively simple and inexpensive manner. However, it would be desirable to apply the same technology to existing electronic devices, the system of the ""848 patent being provided as an add-on apparatus which can be pluggably coupled to a sound receiving device to thereby improve its sound detection characteristic without adding appreciably to its cost and without necessitating re-wiring or other modification of the existing device.
In accordance with the present invention, noise control for a sound receiving device is effected using a system which pluggably mates with the sound receiving device. The system provides a bi-directional microphone having front and back sound-receiving sides. Ambient noise is directed toward the back side and a user""s voice is directed away from the back side using a curved deflector. The user""s voice is also directed away from the back side using a shield element.
Further in accordance with the invention, the system provides a plug which mates with an external microphone input jack of the sound receiving device. The plug establishes electrical contact between the noise control system and the sound receiving device. A resistor for attenuating the signal detected by the bi-directional microphone in order to overcome the effect of signal enhancement expedients of the sound receiving device is connected in series with the bi-directional microphone.